The goal of the proposed research is to investigate the neural about representation of number in infancy. Adult humans -possess sophisticated numerical reasoning abilities, however, it has recently been proposed that non-human animals and human infants may also have a primitive numerical sense. New research suggests that monkeys are capable of comparing numbers, and that chimpanzees can pair abstract symbols with numerosities. Furthermore, human infants spontaneously discriminate between novel and familiar numerical values, even when other stimulus attributes, such as surface area, are held constant. These recent findings have led some researchers to suggest the hypothesis that a primitive numerical processing system evolved in the ancient ancestors of humans and other vertebrates and that system is in place early in human development. The current proposal explores the ontogeny of numerical representations in human infants by testing two predictions of this hypothesis. First, numerical discrimination in infants is characterized by numerical distance and size effects consistent with Weber's Law, Just as it is in adults and nonhuman animals. Second, the same neural architecture supports numerical representations in infants and adults. These hypotheses will be tested by recording Event-Related-Potentials (ERPs) in both adults and infants during passive viewing of numerical stimuli. In the proposed experiments, subjects will first be familiarized with a standard numerical value and then will be shown new values that vary in numerical disparity from the standard. ERPs will be recorded during stimulus presentation and later analyzed as a function of the numerical distance between the numerical standard and test stimuli. Statistical modeling will also be used to make inferences about the sources of electrical activity in both adults and infants. While purely behavioral research has demonstrated that infants can discriminate stimuli based solely on number, the proposed non-invasive neurobiological research will provide new insights into both the format and the neural correlates of numerical representations in infants. Such insights will contribute to a fuller understanding of the development of mathematical thinking in adults, and may also have important implications for teaching elementary mathematics to children.